A reflective display is a non-emissive display in which ambient light for viewing the displayed information is reflected from the display back to the viewer rather than light from behind the display being transmitted through the display. An example of a reflective display is shown in FIG. 1, which depicts a conventional display device 100 formed of a stack of controllable absorption layers used to allow independent control of different colors. More particularly, the conventional display device 100 is formed of a stack of selective absorption layers 108B, 108R, and 108G formed of dye guest/host liquid crystal (LC) layers made to absorb, respectively, blue, red and green light. The blue-absorbing layer 108B is at the top of the stack and the green-absorbing layer 108G is at the bottom of the stack.
Each of the absorption layers 108B, 108R, and 108G is sandwiched between respective pairs of transparent substrates 104 and transparent conductors 106 and are configured to be wholly or partially actuated by the application of suitable electric signals via the conductors 106. Thus, selected pixel regions of each of the absorption layers 108B, 108R, and 108G may be made either to absorb light in a particular wavelength band or substantially to transmit all incident light. The stack also includes a mirror 110W that functions as a broadband reflector that reflects light of all wavelengths back through the layers of the stack to the viewer 112.
As with many color reflective display technologies, the brightness and color gamut of these types of conventional reflective displays is limited, in part, due to absorption and stray reflection in the numerous electrode and substrate layers that are required to achieve full color. For instance, both brightness and contrast are limited by the dichroic ratios of the absorbing species (anisotropic dye), the angular dependence of the incident light, and the degree to which the dichroic absorbers can be oriented by the LC in each of the absorption layers 108B, 108R, and 108G.
This loss in brightness and contrast in conventional reflective displays may be seen graphically in the charts 200 and 220 depicted in FIGS. 2A and 2B, respectively. The chart 200 in FIG. 2A depicts an absorption spectra 202 for an ideal display of colors, in which, the colors, in this case, yellow 204, magenta 206, and cyan 208 are each neatly in their respective wavelength bands and form a top-hat formation. In this ideal case, a yellow layer is made to only absorb blue light, a magenta layer is made to only absorb green light, and a cyan layer is made to only absorb red light.
In real situations, however, the absorption characteristics of light in different wavelength bands 224-228 by the absorption layers 108B, 108R, and 108G is more Gaussian in nature and tend to be much broader on the short wavelength side of the peak than on the long wavelength side, as shown in the chart 220 in FIG. 2B. This means that all of the absorption layers 108B, 108R, and 108G are absorbing some blue light, and the magenta and cyan layers are both absorbing some green light. As a consequence, there is a relatively large amount of losses in the light that enters the stack and is reflected out of the stack.